Graphite crucible for single crystal pulling apparatus and method of manufacturing same

ABSTRACT

A graphite crucible ( 2 ) for retaining a quartz crucible ( 1 ) has a graphite crucible substrate ( 3 ) as a graphite crucible forming material, and a coating film ( 4 ) made of a carbonized phenolic resin and formed over the entire surface of the graphite crucible substrate ( 3 ). The phenolic resin is impregnated inside open pores ( 5 ) existing in a surface of the graphite crucible substrate ( 3 ). The coating film ( 4 ) may be formed only within a portion of the graphite crucible in which SiC formation can occur easily, not over the entirety of the surface of the graphite crucible. For example, it is possible to deposit the film only on the entire inner surface of the crucible. It is also possible to deposit the film only on a curved portion (sharply curved portion) of the inner surface, or only on a curved portion and a straight trunk portion.

TECHNICAL FIELD

The present invention relates to a carbon crucible used for retaining aquartz crucible used in an apparatus for pulling a single crystal ofsilicon or the like by a Czochralski process (hereinafter referred to asa “CZ process”), and to a method of manufacturing the same.

BACKGROUND ART

Single crystals of silicon or the like used for manufacturing ICs andLSIs are usually manufactured by a CZ process. The CZ process is asfollows. Polycrystalline silicon is put in a high-purity quartzcrucible, and while rotating the quartz crucible at a predeterminedspeed, the polycrystalline silicon is heated by a heater to melt thepolycrystalline silicon. A seed crystal (silicon single crystal) isbrought into contact with the surface of the melt, and is graduallypulled up while being rotated at a predetermined speed to solidify thepolycrystalline silicon melt, whereby a silicon single crystal is grown.

However, the quartz crucible softens at high temperature and isinsufficient in strength. For this reason, when in use, the quartzcrucible is usually fitted in a graphite crucible so that the quartzcrucible can be reinforced by being supported by the graphite crucible.

In a crucible apparatus having the quartz crucible and the graphitecrucible as described above, the quartz crucible (SiO₂) and the graphitecrucible (C) react with each other at the fitted surface where they arein contact with each other during high temperature heating, generatingSiO gas. The generated SiO gas reacts with the graphite crucible. Inparticular, while infiltrating the inside of the open pores in thesurface layer portion of the graphite crucible, it reacts with thegraphite crucible (C) and gradually turns the inside of the open poresof the graphite crucible into SiC. Accordingly, when such a heattreatment is carried out repeatedly, the graphite crucible is graduallyturned into SiC, so that the dimensions of the graphite crucible may bechanged, or the graphite crucible may become brittle as a material andmicrocracks develop therein, causing the graphite crucible to break inthe end.

In order to solve such a problem, it has been proposed that a protectivesheet made of an expanded graphite material is interposed between thequartz crucible and the graphite crucible so as to cover the innersurface of the graphite crucible, whereby the SiC formation in thegraphite crucible can be prevented to keep the life of the graphitecrucible long (for example, see Patent Document 1).

CITATION LIST Patent Literature

-   [Patent Document 1] Japanese Patent No. 2528285

SUMMARY OF INVENTION Technical Problem

Nevertheless, in reality, even when the protective sheet is interposedas in the above-described conventional example, the SiC formation in thegraphite crucible cannot be inhibited sufficiently.

Accordingly, there has heretofore been a need for a graphite cruciblefor single crystal pulling apparatus that makes it possible to prolongthe life span.

The present invention has been accomplished in view of the foregoingcircumstances. It is an object of the invention to provide a graphitecrucible for single crystal pulling apparatus and a method ofmanufacturing the same that make it possible to prolong the life span.

Solution to Problem

In order to accomplish the foregoing object, the present inventionprovides a graphite crucible for single crystal pulling apparatuswherein a phenolic resin impregnated in open pores existing in a surfaceof a graphite crucible substrate is carbonized.

With the just-described configuration, the carbonized phenolic resinthat is impregnated into the inner surfaces of a large number of openpores existing in the surface of the graphite crucible substrate caneffectively inhibit the reaction between C and SiO gas over the entiresurface of the graphite crucible substrate, and inhibit development ofthe SiC formation. As a result, the service life of the graphitecrucible can be prolonged.

The formation of the coating film by the carbonized phenolic resin maybe only within a portion of the graphite crucible in which SiC formationcan occur easily, not over the entirety of the surface of the graphitecrucible. For example, it is possible to form the film only on theentire inner surface of the crucible. It is also possible to form thefilm only on a curved portion (sharply curved portion) of the innersurface, or only on the curved portion and a straight trunk portion.

In the present invention, it is preferable that the coating film have anaverage thickness of 10 μm or less. If the thickness of the coating filmexceeds 10 μm, there is a risk that the coating film may be easilypeeled.

The present invention also provides a method of manufacturing a graphitecrucible for single crystal pulling apparatus, characterized bycomprising the steps of: immersing a graphite crucible substrate in aphenolic resin solution under room temperature and normal pressure;curing the phenolic resin by taking out and heat-treating the immersedgraphite crucible substrate; and carbonizing the phenolic resin bysubjecting the cured phenolic resin to a further heat treatment.

The just-described configuration makes it possible to manufacture agraphite crucible in which the phenolic resin is impregnated into theinner surfaces of a large number of open pores existing in the surfaceof the graphite crucible substrate, so that the service life of thegraphite crucible can be prolonged.

In the present invention, it is preferable that the method furthercomprise, prior to the curing step, the step of wiping off an excessiveamount of the phenolic resin on a surface of the graphite cruciblesubstrate.

With the just-described configuration, the surface layer of the graphitecrucible substrate is coated with a necessary amount of the phenolicresin. Therefore, the SiC formation can be effectively prevented.Moreover, it is possible to obtain a graphite crucible that does notchange much in dimensions even after the heat treatment.

In the present invention, it is preferable that the phenolic resinsolution have a viscosity of from 100 mP·s (18° C.) to 400 mP·s (18°C.).

With the just-described configuration, the phenolic resin can beimpregnated sufficiently in the open pores in the graphite cruciblesubstrate. Moreover, an appropriate amount of the resin can be coatedeasily when wiping off an excessive amount of the phenolic resin on thesurface of the graphite crucible substrate. Furthermore, the resincontent is prevented from being squirted out after the heat treatment.

In the present invention, it is preferable that the method furthercomprise, subsequent to the curing step, the step of performing a heattreatment at a temperature equal to or higher than a servicetemperature.

With the just-described configuration, heat-treating at a temperatureequal to or higher than the service temperature serves to stabilize thebonding of the coating film with the substrate, so the film is unlikelyto peel off.

In the present invention, it is preferable that the method furthercomprise, subsequent to the curing step, the step of refining thegraphite crucible substrate on which a coating film of the phenolicresin is formed, by heat-treating the graphite crucible substrate undera halogen gas atmosphere.

With the just-described configuration, the amount of impurities producedfrom the graphite crucible can be reduced, so a high quality metalsingle crystal can be obtained.

In order to accomplish the foregoing object, the present invention alsoprovides a graphite crucible for single crystal pulling apparatus,wherein a coating film of pyrocarbon is formed on an entirety of or aportion of a surface of a graphite crucible substrate, and the coatingfilm is formed so as to reach an inner surface of open pores existing inthe surface of the graphite crucible substrate.

Herein, pyrocarbon (PyC) refers to a high-purity and high-crystallinitygraphitized substance obtained by thermally decomposing a hydrocarbon,for example, a hydrocarbon gas or a hydrocarbon compound having 1 to 8carbon atoms, particularly 3 carbon atoms, to infiltrate and depositinto a deep layer portion of a substrate.

With the just-described configuration, the pyrocarbon is deposited andfilled over the inner surfaces of a large number of open pores existingin the surface of the graphite crucible substrate. As a result, thereaction between C and SiO gas can be effectively inhibited over theentire surface of the graphite crucible substrate, and development ofthe SiC formation can be inhibited. As a result, the service life of thegraphite crucible can be prolonged.

It should be noted that the coating film of pyrocarbon may be formedonly within a portion of the graphite crucible in which SiC formationcan occur easily, not over the entirety of the surface of the graphitecrucible. For example, it is possible to deposit the film only on theentire inner surface of the crucible. It is also possible to deposit thefilm only on a curved portion (sharply curved portion) of the innersurface, or only on the curved portion and a straight trunk portion.

In the present invention, it is preferable that the pyrocarbon coatingfilm have an average thickness of 100 μm or less. If the thicknessexceeds 100 μm, the cost will become high, and an extremely long timetreatment will become necessary to form a pyrocarbon coating film with100 μm or thicker, so the production efficiency decreases.

In the present invention, it is preferable that the coating film beformed by a CVI method.

Herein, the CVI (Chemical Vapor Infiltration) method refers to atechnique for infiltrating and depositing the above-described pyrocarbon(PyC), wherein the reaction process may be conducted as follows: anitrogen gas or a hydrogen gas is used for adjusting the concentrationof a hydrocarbon or a hydrocarbon compound; the hydrocarbonconcentration is set at 3% to 30%, preferably 5% to 15%; and the totalpressure is set at 100 Torr, preferably 50 Torr or less. When such aprocess is carried out, the hydrocarbon forms a giant carbon compound onor near the substrate surface by, for example, dehydrogenation, thermaldecomposition, or polymerization, and the giant carbon compound isdeposited on the graphite crucible substrate; and further thedehydrogenation reaction proceeds, finally forming a dense PyC film fromthe surface of the graphite crucible substrate to the inside thereof.

The temperature range of the deposition is usually wide, from 800° C. to2500° C., but in order to deposit the film into a deep portion of thegraphite crucible substrate, it is desirable that the PyC be depositedin a relatively low temperature region of 1300° C. or lower. Inaddition, it is suitable that the deposition time should be set at along time, at 50 hours, or preferably 100 hours or longer, in order toform a thin PyC of, for example, 100 μm or less. Also, in order toenhance the efficiency in the deposition of pyrocarbon, it is possibleto use what is called an isothermal method, a thermal gradient method, apressure gradient method, a pulse method, or the like, as appropriate.For reference, the CVD (Chemical Vapor Deposition) method is a techniqueof directly depositing decomposed carbon into the texture. Therefore,unlike the CVI method, the CVD method cannot cause decomposed carbon toinfiltrate and form a film inside a substrate, and it can merely depositthick pyrocarbon within a short time.

The present invention also provides a method of manufacturing a graphitecrucible for single crystal pulling apparatus, which comprises the stepof forming a coating film of pyrocarbon by a CVI method so that thecoating film of pyrocarbon is formed on an entirety of or a portion of asurface of a graphite crucible substrate and that the coating film isformed so as to reach an internal surface of open pores existing in asurface of the graphite crucible substrate.

The just-described configuration makes it possible to manufacture agraphite crucible in which the pyrocarbon is impregnated into the innersurfaces of a large number of open pores existing in the surface of thegraphite crucible substrate, so that the service life of the graphitecrucible can be prolonged.

In the present invention, it is preferable that the method furthercomprise the step of refining the graphite crucible substrate on whichthe coating film of the pyrocarbon is formed, by heat-treating thegraphite crucible substrate under a halogen gas atmosphere. The amountof impurities produced from the graphite crucible can be reduced, so ahigh quality metal single crystal can be obtained.

Advantageous Effects of Invention

According to the present invention, the carbonized phenolic resinimpregnated into the inner surfaces of a large number of open poresexisting in the surface of the graphite crucible substrate caneffectively inhibit the reaction between C and SiO gas over the entiresurface of the graphite crucible substrate, thus inhibiting developmentof the SiC formation. As a result, the service life of the graphitecrucible can be prolonged.

Moreover, according to the present invention, the pyrocarbon isdeposited and filled over the inner surfaces of a large number of openpores existing in the surface of the graphite crucible substrate. As aresult, the reaction between C and SiO gas can be effectively inhibitedover the entire surface of the graphite crucible substrate, anddevelopment of the SiC formation can be inhibited. As a result, theservice life of the graphite crucible can be prolonged.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross-sectional view illustrating a graphitecrucible for single crystal pulling apparatus according to Embodiment 1.

FIG. 2 shows partially-enlarged cross-sectional views each illustratinga surface of a graphite crucible substrate according to Embodiment 1.

FIG. 3 is a schematic cross-sectional view illustrating a graphite moldused for fabricating synthetic quartz.

FIG. 4 is a vertical cross-sectional view illustrating a graphitecrucible for single crystal pulling apparatus according to Embodiment 2.

FIG. 5 shows partially-enlarged cross-sectional views each illustratinga surface of a graphite crucible substrate according to Embodiment 2.

FIG. 6 is a view illustrating the position where test sample C is takenin the examples corresponding to Embodiment 1.

FIG. 7 is a graph illustrating the distribution states of pores (openpores) before and after a SiC formation reaction test in an examplecorresponding to Embodiment 1.

FIG. 8 is a photograph illustrating the condition of test sample A(present invention treated product) after ashing subsequent to a SiCformation reaction test in an example corresponding to Embodiment 1.

FIG. 9 is a photograph illustrating the condition of test sample B(present invention treated product) after ashing subsequent to a SiCformation reaction test in an example corresponding to Embodiment 1.

FIG. 10 is a photograph illustrating the condition of test sample A(non-treated product) after ashing subsequent to a SiC formationreaction test in an example corresponding to Embodiment 1.

FIG. 11 is a photograph illustrating the condition of test sample B(non-treated product) after ashing subsequent to a SiC formationreaction test in an example corresponding to Embodiment 1.

FIG. 12 is a SEM photograph of test sample A (present invention treatedproduct) subsequent to a SiC formation reaction test in an examplecorresponding to Embodiment 1.

FIG. 13 is a SEM photograph of test sample B (present invention treatedproduct) subsequent to a SiC formation reaction test in an examplecorresponding to Embodiment 1.

FIG. 14 is a SEM photograph of test sample C (present invention treatedproduct) subsequent to a SiC formation reaction test in an examplecorresponding to Embodiment 1.

FIG. 15 is a SEM photograph of test sample A (non-treated product)subsequent to a SiC formation reaction test in an example correspondingto Embodiment 1.

FIG. 16 is a SEM photograph of test sample C (non-treated product)subsequent to a SiC formation reaction test in an example correspondingto Embodiment 1.

FIG. 17 is a view illustrating the position where test sample C1 istaken in Examples corresponding to Embodiment 2.

FIG. 18 is a graph illustrating the distribution states of pores (openpores) before and after a SiC formation reaction test in an examplecorresponding to Embodiment 2.

FIG. 19 is a photograph illustrating the condition of test sample A1(present invention treated product) after ashing subsequent to a SiCformation reaction test in an example corresponding to Embodiment 2.

FIG. 20 is a photograph illustrating the condition of test sample B1(present invention treated product) after ashing subsequent to a SiCformation reaction test in an example corresponding to Embodiment 2.

FIG. 21 is a photograph illustrating the condition of test sample A1(non-treated product) after ashing subsequent to a SiC formationreaction test in an example corresponding to Embodiment 2.

FIG. 22 is a photograph illustrating the condition of test sample B1(non-treated product) after ashing subsequent to a SiC formationreaction test in an example corresponding to Embodiment 2.

FIG. 23 is a SEM photograph of test sample A1 (present invention treatedproduct) subsequent to a SiC formation reaction test in an examplecorresponding to Embodiment 2.

FIG. 24 is a SEM photograph of test sample B1 (present invention treatedproduct) subsequent to a SiC formation reaction test in an examplecorresponding to Embodiment 2.

FIG. 25 is a SEM photograph of test sample C1 (present invention treatedproduct) subsequent to a SiC formation reaction test in an examplecorresponding to Embodiment 2.

FIG. 26 is a SEM photograph of test sample A1 (non-treated product)subsequent to a SiC formation reaction test in an example correspondingto Embodiment 2.

FIG. 27 is a SEM photograph of test sample C1 (non-treated product)subsequent to a SiC formation reaction test in an example correspondingto Embodiment 2.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, the present invention will be described based on thepreferred embodiments. It should be noted that the present invention isnot limited to the following embodiments.

Embodiment 1

FIG. 1 is a vertical cross-sectional view for illustrating one exampleof a graphite crucible for single crystal pulling apparatus according toEmbodiment 1. A graphite crucible 2 for retaining a quartz crucible 1includes a graphite crucible substrate 3 as a graphite crucible formingmaterial, and a coating film 4 made of a carbonized phenolic resin andformed over the entire surface of the graphite crucible substrate 3(hereinafter the coating film may also be referred to simply as a“phenolic resin coating film”). The graphite crucible substrate 3 usedhere should have a bulk density of 1.70 Mg/m³ or higher, a flexuralstrength of 30 MPa or higher, and a Shore hardness of 40 or higher asits characteristics, in order to ensure necessary mechanical strengthfor a crucible and also taking into consideration readiness of thephenolic resin impregnation. The carbonized substance that constitutesthe coating film 4 may be a graphitized substance the entirety or aportion of which has been subjected to a graphitization process.

Here, the shape of the graphite crucible 2 is generally in a cup-likeshape, formed by a bottom portion 2 a, a curved portion (sharply curvedportion) 2 b curved upward and connected to the bottom portion 2 a, anda straight trunk portion 2 c extending upward straightly and beingconnected to the curved portion 2 b. The shape of the graphite cruciblesubstrate 3 corresponds to the shape of the graphite crucible 2, and itis formed by a bottom portion 3 a, a curved portion (sharply curvedportion) 3 b, and a straight trunk portion 3 c. In the graphite cruciblesubstrate 3 with such a configuration, the phenolic resin coating filmmay be formed either over the entirety of the surface of the graphitecrucible substrate 3 or only within a portion thereof in which SiCformation can occur easily. For example, it is possible to deposit thefilm only on the entire inner surface of the crucible. It is alsopossible to deposit the film only on the curved portion (sharply curvedportion) 3 b of the inner surface, or only on the curved portion 3 b andthe straight trunk portion 3 c.

Next, the condition of the graphite crucible substrate 3 whose surfaceis coated by the phenolic resin coating film 4 will be described withreference to FIG. 2. FIG. 2 shows partially-enlarged cross-sectionalviews illustrating a surface of the graphite crucible substrate 3according to Embodiment 1. FIG. 2( a) schematically shows a condition inwhich the phenolic resin coating film 4 is formed in a desirable mannerover the entire surface of the graphite crucible substrate 3, and FIG.2( b) schematically shows the condition in which the formation thereofis undesirable. The graphite crucible substrate 3 has very small poresin its surface which are called open pores 5. As illustrated in thefigure, the open pores 5 form recesses in the surface. For this reason,the surface area of the graphite crucible substrate 3 is greater thanthat is apparently observed. So, the recess that has a small entrancebut has a large internal space as shown in the figure needs to becovered by impregnating the phenolic resin into the inside of the recessas shown in FIG. 2( a).

For example, when the impregnated phenolic resin covers only the openingportion of the open pore 5 and cannot fill the inside thereof asillustrated in FIG. 2( b), cracks may be caused at the just-mentionedopening portion, which is instable in terms of strength, causing theinside portion that is not coated with the phenolic resin to be exposedto the outside in which SiO gas exists. For this reason, in the presentinvention, the phenolic resin impregnation is carried out under theviscosity, the immersing conditions, and the curing conditions of thephenolic resin solution as follows.

The graphite crucible with the above-described configuration wasproduced in the following manner.

A graphite crucible substrate was immersed in a phenolic resin solutionhaving a viscosity of from 100 mP·s (18° C.) to 400 mP·s (18° C.) underroom temperature and normal pressure for 12 hours or longer. Theimmersed graphite crucible substrate was taken out and heat-treated tocure the phenolic resin, and the cured phenolic resin was subjected to afurther heat treatment to carbonize the phenolic resin.

It is preferable that, prior to the curing step, an excessive amount ofthe phenolic resin on a surface of the graphite crucible substrate bewiped off. By wiping off the phonolic resin, the surface layer of thegraphite crucible substrate is coated with a necessary amount of thephenolic resin. Therefore, the SiC formation can be effectivelyprevented. Moreover, it is possible to obtain a graphite crucible thatdoes not change much in dimensions even after the heat treatment.

It is also preferable that, subsequent to the curing step, the graphitecrucible substrate on which the coating film of the phenolic resin hasbeen formed be heat-treated at a temperature equal to or higher than aservice temperature. The reason is that heat-treating at a temperatureequal to or higher than the service temperature serves to stabilize thebonding of the coating film with the substrate, so the film is unlikelyto peel off.

It is also preferable that, subsequent to the curing step, the graphitecrucible substrate on which the coating film of the phenolic resin isformed be refined by heat-treating the graphite crucible substrate undera halogen gas atmosphere. The reason is that the amount of impuritiesproduced from the graphite crucible can be reduced, so a high qualitymetal single crystal can be obtained.

In the present embodiment, the above-described phenolic resinimpregnating-curing-carbonizing treatment made it possible to obtain agraphite crucible coated with a coating film made of the carbonizedphenolic resin that is sufficiently impregnated into the inside of thesubstrate.

Thus, the carbonized phenolic resin that is impregnated into the innersurfaces of a large number of open pores existing in the surface of thegraphite crucible substrate can effectively inhibit the reaction betweenC and SiO gas over the entire surface of the graphite cruciblesubstrate, and inhibit development of the SiC formation. As a result,the service life of the graphite crucible can be prolonged.

It should be noted that the graphite crucible coated with the phonolicresin should preferably be refined by heat-treating the graphitecrucible substrate under a halogen gas atmosphere. The reason is thatthe amount of impurities produced from the graphite crucible can bereduced, so a high quality metal single crystal can be obtained.

Other Embodiments

In the foregoing embodiment 1, the graphite crucible for single crystalpulling apparatus is the subject of the surface treatment. However, itis also possible to form a coating film made of carbonized phenolicresin on the surface of graphite members used for fabricating syntheticquartz, such as a graphite mold 10, a graphite lid 11, and the like usedfor fabricating synthetic quartz as illustrated in FIG. 3, by using thephenolic resin impregnating-curing-carbonizing treatment as inEmbodiment 1. A conventional problem with the graphite member molds andlids used for fabricating synthetic quartz has been that, when they arein contact with synthetic quartz, the resulting SiO₂ gas promotes SiCformation, which causes dimensional changes and weakening of thematerial, leading to formation of microcracks and finally fractures.However, by forming a coating film of carbonized phenolic resin on thesurface by the phenolic resin impregnating-curing-carbonizing treatment,the SiC formation can be inhibited, and a longer life span can beobtained. Note that in FIG. 3, reference numeral 12 indicates arod-shaped material, reference numerals 13 indicates a heater, referencenumeral 14 indicates an inert gas introducing port, and referencenumeral 15 indicates a gas exhaust port.

Embodiment 2

FIG. 4 is a vertical cross-sectional view for illustrating one exampleof a graphite crucible for single crystal pulling apparatus according toEmbodiment 2. A graphite crucible 2 for retaining a quartz crucible 1includes a graphite crucible substrate 3 as a graphite crucible formingmaterial, and a pyrocarbon coating film 4A formed over the entiresurface of the graphite crucible substrate 3. The graphite cruciblesubstrate 3 used here should have a bulk density of 1.65 Mg/m³ orhigher, a flexural strength of 30 MPa or higher, and a Shore hardness of40 or higher as its characteristics, in order to ensure necessarymechanical strength for a crucible and also taking into considerationreadiness of the deposition of pyrocarbon.

Here, the shape of the graphite crucible 2 is generally in a cup-likeshape, formed by a bottom portion 2 a, a curved portion (sharply curvedportion) 2 b curved upward and connected to the bottom portion 2 a, anda straight trunk portion 2 c extending upward straightly and beingconnected to the curved portion 2 b. The shape of the graphite cruciblesubstrate 3 corresponds to the shape of the graphite crucible 2, and itis formed by a bottom portion 3 a, a curved portion (sharply curvedportion) 3 b, and a straight trunk portion 3 c. In the graphite cruciblesubstrate 3 with such a configuration, the pyrocarbon coating film maybe formed either over the entirety of the surface of the graphitecrucible substrate 3 or only within a portion thereof in which SiCformation can occur easily. For example, it is possible to deposit thefilm only on the entire inner surface of the crucible. It is alsopossible to deposit the film only on the curved portion (sharply curvedportion) 3 b of the inner surface, or only on the curved portion 3 b andthe straight trunk portion 3 c.

Next, the condition of the graphite crucible substrate 3 whose surfaceis coated by the pyrocarbon coating film 4A will be described withreference to FIG. 5. FIG. 5 shows partially-enlarged cross-sectionalviews illustrating a surface of the graphite crucible substrate 3according to Embodiment 2. FIG. 5( a) schematically shows a condition inwhich the pyrocarbon coating film 4A is formed in a desirable mannerover the entire surface of the graphite crucible substrate 3, and FIGS.5( b) and 5(c) schematically show the condition in which the formationthereof is undesirable. The graphite crucible substrate 3 has very smallpores in its surface which are called open pores 5. The open pores 5form recesses in the surface. For this reason, the surface area of thegraphite crucible substrate 3 is greater than that is apparentlyobserved. So, for the recess that has a small entrance but has a largeinternal space as shown in the figure, it is necessary that even theinside of the recess needs to be covered sufficiently by the pyrocarbonfilm as shown in FIG. 5( a).

When the coating film is formed within a short time as in the CVDmethod, only the opening of the open pore is covered as shown in FIG. 5(b), and the inside thereof cannot be coated sufficiently. In this case,there is a risk that cracks may be caused at the just-mentioned openingportion, which is instable in terms of strength, causing the insideportion that is not coated with the pyrocarbon film to be exposed to theoutside in which SiO gas exists. Or, even though the opening portion ofthe open pore 5 may not be closed, the inside of the open pore 5 cannotbe coated sufficiently as shown in FIG. 5( c), and the portion that isnot coated with the pyrocarbon film is exposed to the outside in whichSiO gas exists, as in the just-described case. Accordingly, in order tosufficiently coat the graphite crucible substrate 3 in which a largenumber of open pores exist in its surface, it is necessary to slow downthe deposition rate of the pyrocarbon film so that the pyrocarbon filmcan be deposited into the inside of the open pores. From such aviewpoint, it is desirable that the deposition rate of the pyrocarbonfilm be 0.2 μm/h or lower. The above-described CVI method is suitablefor obtaining a thin pyrocarbon film with such a slow deposition rate.

In the present embodiment, the use of the above-described CVI methodmade it possible to obtain a graphite crucible coated with a pyrocarboncoating film that is sufficiently impregnated into the inside of thesubstrate.

Thus, the pyrocarbon is deposited and filled over the inner surfaces ofa large number of open pores existing in the surface of the graphitecrucible substrate. As a result, the reaction between C and SiO gas canbe effectively inhibited over the entire surface of the graphitecrucible substrate, and development of the SiC formation can beinhibited. As a result, the service life of the graphite crucible can beprolonged.

It should be noted that the graphite crucible coated with the pyrocarboncoating film should preferably be refined by heat-treating the graphitecrucible substrate under a halogen gas atmosphere. The reason is thatthe amount of impurities produced from the graphite crucible can bereduced, so a high quality metal single crystal can be obtained.

Other Embodiments

In the foregoing embodiment 2, the graphite crucible for single crystalpulling apparatus is the subject of the surface treatment. However, itis also possible to form a pyrocarbon coating film on the surface ofgraphite members used for fabricating synthetic quartz, such as agraphite mold 10, a graphite lid 11, and the like used for fabricatingsynthetic quartz as illustrated in FIG. 3, by using the CVI method as inEmbodiment 2. A conventional problem with the graphite member molds andlids used for fabricating synthetic quartz has been that, when they arein contact with synthetic quartz, the resulting SiO₂ gas promotes SiCformation, which causes dimensional changes and weakening of thematerial, leading to formation of microcracks and finally fractures.However, by forming a pyrocarbon coating film on the surface by the CVImethod, the SiC formation can be inhibited, and a longer life span canbe obtained.

EXAMPLES

Hereinbelow, the present invention will be described in detail byexamples. It should be noted that the present invention is in no waylimited to the following examples.

Examples Corresponding to Embodiment 1 Test Example 1

Dimensional changes were investigated for the following test samples.

(Test Sample)

A graphite material was surface-treated by the same phenolic resinimpregnating-curing-carbonizing treatment as described in the foregoingembodiment 1. For two kinds of graphite materials, the surface-treatedgraphite material and a non-treated graphite material, samples with thefollowing shape were prepared for testing.

Divided pieces of 3-piece graphite crucible: 1 piece for each

Hereinbelow, a divided piece using the surface-treated graphite materialis referred to as a present invention treated product, and a dividedpiece using the non-treated graphite material is referred to as anon-treated product.

(Phenolic Resin Impregnating-Curing-Carbonizing Treatment)

The phenolic resin impregnating and curing treatment was carried out inthe following manner.

The viscosity of the phenolic resin solution used: 195 mP·s (18° C.)

Immersing conditions: Test samples were immersed in the just-mentionedphenolic resin solution at room temperature and normal pressure for 24hours.

Curing conditions: The temperature was elevated to 200° C. gradually soas not to foam, and thereafter kept at 200° C. for curing.

Note that the test samples after curing was heated under a halogen gasatmosphere at 2000° C. to perform a refining process (which correspondsto the carbonizing treatment for the phenolic resin).

(Test Results)

The dimensional changes in height, inner diameters at 50 mm and 150 mmfrom the upper end of the crucible, and radius of the sharply curvedportion were investigated for the present invention treated product andthe non-treated product. The results are shown in Table 1.

TABLE 1 Non treated product Present invention treated product Size SizeVariation Change ratio mm mm mm % Height 330.01 330.18 0.17 0.05 Innerdiameter 459.08 459.32 0.24 0.05 (50 mm from upper end of crucible)Inner diameter 459.12 459.28 0.16 0.04 (150 mm from upper end ofcrucible) Side face sharply 120.00 120.00 0 0 curved portion (radius)

(Evaluation of the Test Results)

As is clear from Table 1, it was confirmed that the present inventiontreated product shows extremely small dimensional changes and that thereis no problem at all in practical use.

Test Example 2

A SiC formation reaction test was conducted for the following testsamples to investigate changes in their physical properties (bulkdensity, hardness, electrical resistivity, flexural strength, and pore(open pore) distribution) before and after the SiC reaction.

(Test Sample)

Two kinds of samples, a present invention treated product and anon-treated product that were the same as those in Test Example 1 exceptfor their shapes, were prepared as the test samples.

The samples with the following shapes were used as the test samples.

Rod-shaped sample with dimensions 10×10×60 (mm): Hereinbelow, thisrod-shaped sample is referred to as test sample A.

Plate-shaped sample with dimensions 100×200×20 (mm): Hereinbelow, thisplate-shaped sample is referred to as test sample B.

A cut-out piece obtained by cutting out a test specimen with dimensions100×20×thickness 20 (mm) from test sample B: (as illustrated in FIG. 6,out of six surfaces thereof, four surfaces are coated surfaces, and theremaining two surfaces are non-coated surfaces): Hereinbelow, thiscut-out piece is referred to as test sample C.

Test samples A and B are also used as the samples for later-describedTest Examples 3 and 4, in addition to for this Test Example 2, and testsample C is used only for the observation by scanning electronmicroscope (SEM) in the later-described Test Example 4.

Of test samples A to C, ones that are surface-treated by the phenolicresin impregnating-curing-carbonizing treatment are referred to aspresent invention treated products, and ones that are notsurface-treated are referred to as non-treated products.

(SiC Formation Reaction Test)

Test samples A to C were subjected to a high-temperature heat treatmentwith synthetic quartz (high purity SiO₂) to compare SiC formationreactivity. The specific conditions in this case are as follows.

Treating furnace: Vacuum furnace

Treatment temperature: 1600° C.

Furnace internal pressure: 10 Torr

Treatment gas: Ar 1 mL/min

Treatment time Retained for 8 hours

Treatment method: Test samples are buried in synthetic quartz powder andheat-treated.

(Test Results)

The physical properties (bulk density, hardness, electrical resistivity,and flexural strength) were studied before and after the surfacetreatment. The results of the measurement for test sample A are shown inTable 2, and the results of the measurement for test sample B are shownin Table 3. The results of the measurement for pore (open pore)distribution are shown in FIG. 5.

TABLE 2 Present invention treated product Non-treated product Bulkdensity 1.79 1.74 (Mg/m³) Hardness 62 55 (HSD) Electrical resistivity12.5 14.0 (μΩm) Flexural strength 52 40 (MPa)

TABLE 3 Present invention treated product Non-treated product Bulkdensity 1.76 1.75 (Mg/m³)

(Evaluation of the Test Results)

As is clear from Tables 2 and 3, the present invention treated productsshow improvements in all of bulk density, hardness, and flexuralstrength over the non-treated products, so it is demonstrated that adensity increase and a strength increase are achieved. Because thesample sizes were different between those in Table 2 and those in Table3, it was confirmed that there were differences in bulk density valuesbetween those in Table 2 and those in Table 3.

In addition, pore (open pore) distribution was studied as the physicalproperties before and after the surface treatment. The results of themeasurement are shown in FIG. 7. The measurement method was as follows.A test specimen for the measurement was taken at about 2.4 mm inthickness from the surface layer of the present invention treatedproduct, and the measurement was conducted for this test specimen formeasurement.

In FIG. 7, L1 represents the distribution for the present inventiontreated product, and L2 represents the distribution for the non-treatedproduct. As is clear from FIG. 7, the present invention treated productwas smaller in volumetric capacity of the pores.

Test Example 3

Mass changes and volumetric changes before and after the SiC reactionwere investigated for test samples A and B that were subjected to theSiC formation reaction test of the foregoing Test Example 2.

(Test Results)

The results of the measurement of mass changes and volumetric changesbefore and after the SiC reaction test are shown in Table 4 below.

TABLE 4 Present invention treated product Non-treated product 10 × 100 ×10 × 100 × 10 × 60 200 × 20 10 × 60 200 × 20 (mm) (mm) (mm) (mm) Masschange ratio −4.9 −1.0 −4.4 −0.9 (%) Volumetric change ratio −4.3 −0.9−5.0 −1.8 (%)

(Evaluation of the Test Results)

As clearly seen from Table 4, it is observed that, in terms of masschange ratio, the non-treated products showed lower mass decreases thanthe present invention treated products, irrespective of the sizes of thesamples. In addition, in terms of volumetric change ratio, the presentinvention treated products showed lower values than the non-treatedproducts. The reactivity cannot be evaluated unconditionally based onthe mass change ratio and the volumetric change ratio because athickness reduction due to the reaction and a mass increase due to theSiC formation occur before and after the test. However, from theresults, it is believed that the phenolic resin impregnating and curingtreatment had the effect of inhibiting the SiC formation. In particular,considerable differences were not observed because the treatment timewas a short time, 8 hours. However, it is believed that if the treatmenttime is set at about 100 hours, considerable differences will beobserved and definitive evaluation will be made.

Test Example 4

For test samples A to C that were subjected to the SiC reaction test inthe same manner as in the foregoing Test Example 4, the thickness of theSiC layer after the reaction test was observed in the following twokinds of methods, (1) observation after ashing and (2) observation byscanning electron microscope.

(1) Observation after Ashing

Using test samples A and B after the SiC reaction test, the remainingportion of the graphite material was incinerated and ashed under the airatmosphere at 800° C., and the thickness of the remaining SiC layer wasinvestigated. The results are shown in Table 5. In addition, theconditions of test samples A and B after ashing are shown in FIGS. 8 to11. Note that FIG. 8 is a photograph illustrating the condition of testsample A (present invention treated product) after ashing, FIG. 9 is aphotograph illustrating the condition of test sample B (presentinvention treated product) after ashing, FIG. 10 is a photographillustrating the condition of test sample A (non-treated product) afterashing, and FIG. 11 is a photograph illustrating the condition of testsample B (non-treated product) after ashing.

TABLE 5 Present invention treated product Non-treated product 100 × 100× 10 × 10 × 60 200 × 20 10 × 10 × 60 200 × 20 (mm) (mm) (mm) (mm)Maximum 0.3 0.8 0.6 1.7 SiC layer thickness (mm) Average 0.3 0.6 0.6 1.0SiC layer thickness (mm)

(Evaluation of the Test Results)

As is clear from FIGS. 8 to 11 and Table 5, it is observed that thepresent invention treated products have greater effects of inhibitingSiC formation than the non-treated products. Although there aredifferences in the SiC layer values depending on the sample size, thepresent invention treated products had about 50% thinner SiC layers ofthose of the non-treated products.

(2) Observation by Scanning Electron Microscope (SEM)

The SEM photographs concerning the surface conditions of test samples Ato C after the SiC reaction test are shown in FIGS. 12 to 16. Note thatFIG. 12 is a SEM photograph of test sample A (present invention treatedproduct), FIG. 13 is a SEM photograph of test sample B (presentinvention treated product), FIG. 14 is a SEM photograph of test sample C(present invention treated product), FIG. 15 is a SEM photograph of testsample A (non-treated product), and FIG. 16 is a SEM photograph of testsample C (non-treated product). In FIGS. 12 to 16, the brace “}”indicates a SiC layer.

(Evaluation of the Test Results)

From the SEM photographs, the thickness of the SiC layer showed the sametendency as the results in ashing. It was confirmed that the presentinvention treated products have advantageous effects of inhibiting SiCformation over the non-treated products.

Examples Corresponding to Embodiment 2 Test Example 1

Dimensional changes were investigated for the following test samples.

(Test Sample)

A graphite material was surface-treated by the same CVI method asdescribed in the foregoing embodiment 2. For two kinds of graphitematerials, this surface-treated graphite material and a non-treatedgraphite material, samples with the following shape were prepared fortesting.

Divided pieces of 3-piece graphite crucible: 1 piece for eachHereinbelow, a divided piece using the surface-treated graphite materialis referred to as a present invention treated product, and a dividedpiece using the non-treated graphite material is referred to as anon-treated product.

(CVI Process)

The CVI process was carried out in the following manner. Specifically,the graphite material was placed in a vacuum furnace and the temperaturewas elevated to 1100° C. Thereafter, while CH₄ gas was being flowed at aflow rate 10 (L/min), the pressure was controlled to be 10 Torr and keptfor 100 hours.

(Test Results)

The dimensional changes in height, inner diameters at 50 mm and 150 mmfrom the upper end of the crucible, and radius of the sharply curvedportion were investigated for the present invention treated product andthe non-treated product. The results are shown in Table 6.

TABLE 6 Non treated product Present invention treated product Size SizeVariation Change ratio mm mm mm % Height 330.01 330.04 0.03 0.01 Innerdiameter 459.08 459.13 0.05 0.01 (50 mm from upper end of crucible)Inner diameter 459.12 459.17 0.05 0.01 (150 mm from upper end ofcrucible) Side face sharply 120.00 120.03 0.03 0.03 curved portion(radius)

(Evaluation of the Test Results)

As is clear from Table 6, it was confirmed that the present inventiontreated product shows extremely small dimensional changes and that thereis no problem at all in practical use.

Test Example 2

A SiC formation reaction test was conducted for the following testsamples to investigate changes in their physical properties (bulkdensity, hardness, electrical resistivity, flexural strength, and pore(open pore) distribution) before and after the SiC reaction.

(Test Sample)

Two kinds of samples, a present invention treated product and anon-treated product that were the same as those in Test Example 1 exceptfor their shapes, were prepared as the test samples.

The samples with the following shapes were used as the test samples.

Rod-shaped sample with dimensions 10×10×60 (mm): Hereinbelow, thisrod-shaped sample is referred to as test sample A1.

Plate-shaped sample with dimensions 100×200×20 (mm): Hereinbelow, thisplate-shaped sample is referred to as test sample B1.

A cut-out piece obtained by cutting out a test specimen with dimensions100×20×thickness 20 (mm) from test sample B1: (as illustrated in FIG.17, out of six surfaces thereof, four surfaces are coated surfaces, andthe remaining two surfaces are non-coated surfaces): Hereinbelow, thiscut-out piece is referred to as test sample C1.

Test samples A1 and B1 are also used as the samples for later-describedTest Examples 3 and 4, in addition to for this Test Example 2, and testsample C1 is used only for observation by scanning electron microscope(SEM) in the later-described Test Example 4.

Of test samples A1 to C1, ones that are surface-treated by the CVImethod are referred to as present invention treated products, and onesthat are not surface-treated are referred to as non-treated products.

(SiC Formation Reaction Test)

Test samples A to C were subjected to a high-temperature heat treatmentwith synthetic quartz (high purity SiO₂) to compare SiC formationreactivity. The specific conditions in this case are as follows.

Treating furnace: Vacuum furnace

Treatment temperature: 1600° C.

Furnace internal pressure: 10 Torr

Treatment gas: Ar 1 mL/min

Treatment time: Retained for 8 hours

Treatment method: Test samples are buried in synthetic quartz powder andheat-treated.

(Test Results)

The physical properties (bulk density, hardness, electrical resistivity,and flexural strength) of test samples A1 and B1 were studied before andafter the surface treatment. The results of the measurement are shown inTables 7 and 8. The results of the measurement for pore (open pore)distribution are shown in FIG. 18.

TABLE 7 Present invention treated product Non-treated product Bulkdensity 1.77 1.74 (Mg/m³) Hardness 65 55 (HSD) Electrical resistivity13.3 14.0 (μΩm) Flexural strength 45 40 (MPa)

TABLE 8 Present invention treated product Non-treated product Bulkdensity 1.76 1.75 (Mg/m³)

(Evaluation of the Test Results)

As is clear from Tables 7 and 8, the present invention treated productsshow improvements in all of bulk density, hardness, and flexuralstrength over the non-treated products, so it is demonstrated that adensity increase and a strength increase are achieved. Because thesample sizes were different between those in Table 2 and those in Table3, it was confirmed that there were differences in bulk density valuesbetween those in Table 2 and those in Table 3.

In addition, pore (open pore) distribution was studied as the physicalproperties before and after the surface treatment. The results of themeasurement are shown in FIG. 18. The measurement method was as follows.A test specimen for the measurement was taken at about 2.4 mm inthickness from the surface layer of the present invention treatedproduct, and the measurement was conducted for this test specimen formeasurement.

In FIG. 18, L3 represents the distribution for the present inventiontreated product, and L4 represents the distribution for the non-treatedproduct. As is clear from FIG. 18, the present invention treated productmade the volumetric capacity of large pores smaller. The CVI made thesize of the pores smaller.

Test Example 3

Mass changes and volumetric changes before and after the SiC reactionwere investigated for test samples A1 and B1 that were subjected to theSiC formation reaction test of the foregoing Test Example 2.

(Test Results)

The results of the measurement of mass changes and volumetric changesbefore and after the SiC reaction test are shown in Table 9 below.

TABLE 9 Present invention treated product Non-treated product 100 × 100× 10 × 10 × 60 200 × 20 10 × 10 × 60 200 × 20 (mm) (mm) (mm) (mm) Masschange ratio −5.0 −1.3 −4.4 −0.9 (%) Volumetric change −5.0 −1.0 −5.0−1.8 ratio (%)

(Evaluation of the Test Results)

As clearly seen from Table 9, it is observed that, in terms of masschange ratio, the non-treated products showed less mass decreases thanthe present invention treated products, irrespective of the sizes of thesamples. In addition, in terms of volumetric change ratio, the presentinvention treated products showed lower values than the non-treatedproducts. The reactivity cannot be evaluated unconditionally based onthe mass change ratio and the volumetric change ratio because athickness reduction due to the reaction and a mass increase due to theSiC formation occur before and after the test. However, from theresults, it is believed that the CVI process had the effect ofinhibiting the SiC formation. In particular, considerable differenceswere not observed because the treatment time was a short time, 8 hours.However, it is believed that if the treatment time is set at about 100hours, considerable differences will be observed and definitiveevaluation will be made.

Test Example 4

For test samples A1 to C1 that were subjected to the SiC reaction testin the same manner as in the foregoing Test Example 4, the thickness ofthe SiC layer after the reaction test was observed in the following twokinds of methods, (1) observation after ashing and (2) observation byscanning electron microscope.

(1) Observation after Ashing

The remaining portions of the graphite material in test samples A and Bafter the SiC reaction test were incinerated and ashed under the airatmosphere at 800° C., and the thickness of the remaining SiC layer wasinvestigated. The results are shown in Table 10. In addition, theconditions of test samples A1 and B1 after ashing are shown in FIGS. 19to 22. Note that FIG. 19 is a photograph illustrating the condition oftest sample A1 (present invention treated product) after ashing, FIG. 20is a photograph illustrating the condition of test sample B1 (presentinvention treated product) after ashing, FIG. 21 is a photographillustrating the condition of test sample A1 (non-treated product) afterashing, and FIG. 22 is a photograph illustrating the condition of testsample B1 (non-treated product) after ashing.

TABLE 10 Present invention treated product Non-treated product 100 × 100× 10 × 10 × 60 200 × 20 10 × 10 × 60 200 × 20 (mm) (mm) (mm) (mm)Maximum 0.4 1.1 0.6 1.7 SiC layer thickness (mm) Average 0.4 0.5 0.6 1.0SiC layer thickness (mm)

(Evaluation of the Test Results)

As is clear from FIGS. 19 to 22 and Table 10, it is observed that thepresent invention treated products have greater effects of inhibitingSiC formation than the non-treated products. Although there aredifferences in the SiC layer values depending on the sample size, thepresent invention treated products had about 50% thinner SiC layers ofthose of the non-treated products.

(2) Observation by Scanning Electron Microscope (SEM)

The SEM photographs concerning the surface conditions of test samples A1to C1 after the SiC reaction test are shown in FIGS. 23 to 27. Note thatFIG. 23 is a SEM photograph of test sample A1 (present invention treatedproduct), FIG. 24 is a SEM photograph of test sample B1 (presentinvention treated product), FIG. 25 is a SEM photograph of test sampleC1 (present invention treated product), FIG. 26 is a SEM photograph oftest sample A1 (non-treated product), and FIG. 27 is a SEM photograph oftest sample C1 (non-treated product). In FIGS. 23 to 27, the brace “}”indicates a SiC layer.

(Evaluation of the Test Results)

From the SEM photographs, the thickness of the SiC layer showed the sametendency as the results in ashing. It was confirmed that the presentinvention treated products have advantageous effects over thenon-treated products.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a graphite crucible for singlecrystal pulling apparatus, and to a method of manufacturing thecrucible.

REFERENCE SIGNS LIST

-   -   1—Quartz crucible    -   2—Graphite crucible    -   3—Graphite crucible substrate    -   4—Phenolic resin coating film    -   4A—Pyrocarbon coating film    -   5—Open pore

1-12. (canceled)
 13. A graphite crucible for single crystal pullingapparatus, characterized in that a phenolic resin impregnated in openpores existing in a surface of a graphite crucible substrate iscarbonized.
 14. The graphite crucible for single crystal pullingapparatus according to claim 13, wherein a coating film of thecarbonized phenolic resin has an average thickness of 10 μm or less. 15.A method of manufacturing a graphite crucible for single crystal pullingapparatus, characterized by comprising the steps of: immersing agraphite crucible substrate in a phenolic resin solution under roomtemperature and normal pressure; curing the phenolic resin by taking outand heat-treating the immersed graphite crucible substrate; andcarbonizing the phenolic resin by subjecting the cured phenolic resin toa further heat treatment.
 16. The method of manufacturing a graphitecrucible for single crystal pulling apparatus according to claim 15,further comprising, prior to the curing step, the step of wiping off anexcessive amount of the phenolic resin on a surface of the graphitecrucible substrate.
 17. The method of manufacturing a graphite cruciblefor single crystal pulling apparatus according to claim 16, wherein thephenolic resin solution has a viscosity of from 100 mPa·s (18° C.) to400 mPa·s (18° C.).
 18. The method of manufacturing a graphite cruciblefor single crystal pulling apparatus according to claim 15, furthercomprising, subsequent to the curing step, the step of performing a heattreatment at a temperature equal to or higher than a servicetemperature.
 19. The method of manufacturing a graphite crucible forsingle crystal pulling apparatus according to claim 15, furthercomprising, subsequent to the curing step, the step of refining thegraphite crucible substrate on which a coating film of the phenolicresin is formed, by heat-treating the graphite crucible substrate undera halogen gas atmosphere.
 20. A graphite crucible for single crystalpulling apparatus, characterized in that a coating film of pyrocarbon isformed on an entirety of or a portion of a surface of a graphitecrucible substrate, and the coating film is formed so as to reach aninner surface of open pores existing in the surface of the graphitecrucible substrate.
 21. The graphite crucible for single crystal pullingapparatus according to claim 20, wherein the coating film has an averagethickness of 100 μm or less.
 22. The graphite crucible for singlecrystal pulling apparatus according to claim 20, wherein the coatingfilm is formed by a CVI method.
 23. The graphite crucible for singlecrystal pulling apparatus according to claim 21, wherein the coatingfilm is formed by a CVI method.
 24. A method of manufacturing a graphitecrucible for single crystal pulling apparatus, characterized bycomprising the step of forming a coating film of pyrocarbon by a CVImethod so that the coating film of pyrocarbon is formed on an entiretyof or a portion of a surface of a graphite crucible substrate and thatthe coating film is formed so as to reach an internal surface of openpores existing in a surface of the graphite crucible substrate.
 25. Themethod of manufacturing a graphite crucible for single crystal pullingapparatus according to claim 24, further comprising the step of refiningthe graphite crucible substrate on which the coating film of pyrocarbonis formed by the pyrocarbon coating film formation step, byheat-treating the graphite crucible substrate under a halogen gasatmosphere.